1. Field of the Invention
The present invention relates to a finder indication optical system which can be advantageously used for an indicator within a finder of a camera, and in particular a small camera, such as a digital camera.
2. Description of the Related Art
In a view finder of a conventional camera, in particular a single lens reflex camera, an indication device is provided in the vicinity of an image surface (focusing screen) of an objective optical system (photographing lens) or provided in a position optically equivalent to the image surface, so that together with an object image formed by the objective optical system, the indication of the indication device can be viewed outside of the photographing area which defines the periphery of a picture to be exposed on a film.
In a view finder of a single lens reflex digital camera or a real-image type view finder of a compact camera in which the image of the objective optical system is smaller than that in a single lens reflex camera using a silver-halide film, an eyepiece (eyepiece optical system) provided within the finder optical system to view an object image formed by the objective optical system is small and has a large magnification. However, the number of the lenses of the eyepiece is decreased in order to reduce the manufacturing cost, and consequently it is difficult to maintain a good image peripheral portion of the field of view. Nevertheless, if the object image formed by the objective optical system is viewed through the eyepiece, the deterioration of the image quality peripheral portion of the is not serious because, normally, a viewer tends not to focus on the peripheral portion of the photographing area. However, in the indication within the finder in which only the peripheral portion of the field of view is utilized, if an indication of high contrast is viewed, deterioration of a finder indication image in the meridional direction or sagittal direction due to aberration and, in particular, astigmatic difference is apparent.
In order to solve the above-mentioned problem of the prior art, it is an object of the present invention to provide an optical system in which an astigmatic difference correction device is provided in a light path of a finder indication optical system to reduce or eliminate distortion of a finder indication image, so that an image of high quality can be viewed within the finder.
To achieve the object, according to the present invention, a finder indication optical system is provided, in which an indication device is provided in the vicinity of an image surface of an objective optical system or provided in a position in which the diopter is substantially equivalent to the image surface of the objective optical system when viewed through a finder field of view of a finder optical system, so that photographing information formed by the indication device can be viewed outside of the photographing area which is formed to be seen in the finder field of view, said indication optical system comprising: a correction surface which is provided along a light path of the finder indication optical system to correct an astigmatic difference thereof,
wherein the correction surface is provided at a predetermined position from the optical axis of the finder optical system as the origin point, and
wherein the correction surface is formed in the following manner:
the optical axis defines an X-axis;
the direction parallel to the transverse direction of said correction surface and perpendicular to the X-axis, defines a Y-axis;
the direction parallel to the longitudinal direction of said correction surface and perpendicular to the X-axis, defines a Z-axis;
the coordinates of the center of said correction surface are (xcex1,xcex2); and
the absolute value of curvature C in a section parallel with a plane defined by said X-axis and said Z-axis, at the coordinates (y, z) of the correction surface, is |Cz(y, z)|;
wherein said correction surface is formed so that the absolute value |CZ(y, z)| of the curvature C increases as the absolute value of z at y=xcex1 increases. Preferably, the absolute value |CY(y, z)| of the curvature C in a section parallel with a plane defined by the X-axis and the Y-axis, at the coordinates (y, z) of the correction surface, satisfies the following condition (1) at y=xcex1 and z=xcex2:
|CY(y, z)|/|CZ(y, z)|xe2x89xa71xe2x80x83xe2x80x83(1) 
Preferably, the following condition (2) is satisfied:
S/fr less than 0.5xe2x80x83xe2x80x83(2); 
wherein fr designates the combined focal length of the optical system which constitutes the finder optical system provided optically behind the correction surface; and S designates the reduced spatial distance between a indication surface of the indication device and the correction surface.
Preferably, the following condition (3) is satisfied:
0.1 less than S/frxe2x80x83xe2x80x83(3). 
In an embodiment, the finder indication optical system includes a prism, wherein the correction surface is formed on at least one of an incidence surface, a reflection surface and an emission surface, of the prism.
In an embodiment, the correction surface is defined by a part of a rotationally symmetric aspherical surface whose rotational axis is to the same as the optical axis of the finder optical system.
In an embodiment, the finder indication optical system includes a real-image finder in which an object image formed by a photographing lens on a primary imaging plane is formed and viewed on a secondary imaging plane through a condenser lens, a relay lens system, and an eyepiece lens system.
In an embodiment, an indicator is further provided, which is located in the vicinity of the primary imaging plane and outside of the light path of the finder optical system, so that the indication surface thereof is located at a position optically equivalent to the primary imaging plane, wherein the prism is provided between the primary imaging plane and the condenser lens, and in the vicinity of the upper portion of the primary imaging plane.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2000-47468 (filed on Feb. 24, 2000) which is expressly incorporated herein by reference in its entirety.